1. Field of the Invention
The present invention relates to an optical apparatus for detecting the position of an object by sensing the light from the object through an objective lens. More specifically, the present invention relates to an optical position-detecting apparatus of the type in which a laser beam is projected onto an object to be examined through an objective lens and the scattered or diffracted light from the object is received to detect the position of the object.
2. Related Background Art
In the known projection type exposure apparatus now used in the manufacture of semiconductor elements such as LSI, an optical alignment system is needed to align a reticle (or a mask) with a wafer very precisely. An optical alignment system making use of laser beam has been known and widely used for this purpose.
In this alignment optical system according to the prior art, the position of a wafer as an object to be examined is detected in the following manner:
A laser beam is focused on the wafer. The focused light is reflected, scattered or diffracted by the surface of the wafer having an alignment mark preformed. The reflected, scattered or diffracted light is received through a spatial filter disposed at the pupil of an objective lens or at a position conjugate with the pupil of the objective. The received light contains information of the position of the alignment mark. The position of the alignment mark is detected as a position on a coordinate system orthogonal to the optical axis of the objective lens to know the exact position of the wafer.
The function of the spatial filter used in the above apparatus is to cut off the normal reflection light from the object surface and to transmit only the scattered light or the diffracted light derived from the alignment mark. Through the spacial filter it is possible to detect the alignment mark in a dark view field and, therefore, it is possible to improve the accuracy in the detection of position.
However, the prior art apparatus involves a problem of noise. A laser beam usually contains not only parallel rays to the optical axis but also some oblique rays which have a small angle of divergence relative to the optical axis (hereinafter, such oblique rays of the laser are referred to as the divergent rays). This is true also for gas laser which is generally regarded as a beam of parallel rays. These oblique rays produce a halo, that is, an area lightly blurred with light surrounding the entrance pupil of the objective lens. Because of this phenomenon, when the light from the object is filtered at the entrance pupil or at a conjugated point with it, the divergent rays form noise which disturbs the correct detection of object position.